This is the "check before you write" skill. It fights the fundamental AI problem: models don't copy-paste, they reimagine, which means you end up with five different date formatters that all do roughly the same thing. The fix is systematic. You maintain a CODE_INDEX.md that catalogs what already exists by capability, not location. Before Claude writes any new utility function, it checks the index, searches for similar code, and either extends what's there or documents why something new is needed. The skill includes the index format (capability tables with function signatures), file header templates, and a step-by-step lookup process. Honestly most useful in established codebases where duplication has already become a tax.
npx skills add https://github.com/alinaqi/claude-bootstrap --skill code-deduplication